1. Field of the Invention
The present invention relates to a new combustion system for gas turbines which, by using additional burners reducing the quantity of additional fuel necessary for the stabilization of the flame and enabling the exact quantity of air and fuel used by the burners themselves to be known, permits not only an excellent and safe ignition of the flame in the combustion chamber i.e. an instantaneous ignition and therefore without pressure waves, but above all a drastic minimization of polluting emissions of nitrogen oxide at all the charge levels of the turbine.
More specifically, the present invention relates to a perfected combustion system with low polluting emissions for gas turbines, as described in Italian patent application MI92 A 002189 filed on Sep. 24, 1992 by the same Applicant.
2. Discussion of the Background
The above patent application relates to a combustion system for a gas turbine, of the pre-mixing type i.e. wherein before the combustion chamber, and separated therefrom by a choke, a pre-mixing chamber is used which, together with the combustion chamber, is surrounded by an air space under circulating pressure countercurrent to the flow of combustion products leaving said combustion chamber, this air being used as combustion air to be mixed with the fuel in the pre-mixing chamber and as cooling air both for the combustion chamber and combustion products. Subsequently, in order to have low polluting emissions of nitrogen oxide at all charge levels of the turbine, in the above known combustion system the passage of combustion air from said air space to the pre-mixing chamber, through windows present in the external surface of the latter, is divided in relation to the quantity of fuel used in order to maintain the ratio combustion air/fuel at the optimum value; in addition, the cooling air of the tapered head and part of the combustion chamber which is immediately after said choke, is sent down into a cooling chamber which communicates with said combustion chamber by means of collector holes situated in the wall of the combustion chamber itself, far away from the choke. On the other hand, to avoid extinguishment or instability of the flame, an anular series of small holes is situated in the surface of said choke for an additional injection of fuel necessary for enriching the combustion area immediately after said choke with fuel.
It has now been experimentally observed that this known combustion system, even if it is capable of considerably reducing the polluting emissions of nitrogen oxide with respect to the traditional systems, continues, in fact, to produce polluting emissions, most of which can be basically attributed to the concentrated injection of additional fuel into the combustion area immediately after the choke, through said anular series of small holes situated in the surface of the choke itself. In fact, by reducing the quantity of fuel injected into this area, there is a considerable reduction of nitrogen oxide.
As the above reduction, however, obviously cannot be prolonged over a certain limit without jeopardizing the stability of the flame, it is evident that a combustion system like the one described, is absolutely unable to minimize the polluting emissions of nitrogen oxide to the extreme.